In the oil & gas industry natural gas is produced, processed and transported to its end-users.
Gas processing may include the liquefaction of at least part of the natural gas stream. If a natural gas stream is liquefied then a range of so called Natural Gas Liquids (NGL's) is obtained, comprising Liquefied Natural Gas or LNG (which predominantly comprises methane or (C1 or CH4), Ethane (C2), Liquefied Petrol Gas or LPG (which predominantly comprises propane and butane or C3 and C4) and Condensate (which predominantly comprise C5+ fractions).
If the gas is produced and transported to regional customers via a pipe-line (grid), the heating value of the gas is limited to specifications. For the richer gas streams this requires midstream processing to recover C2+ liquids, which are sold as residual products.
If regional gas production outweighs regional gas consumption, expensive gas transmission grids cannot be justified, hence the gas may be liquefied to LNG, which can be shipped as bulk. In producing C1 liquids, C2+ liquids are produced concurrently and sold as by-products.
Traditional NGL recovery plants are based on cryogenic cooling processes as to condense the light ends in the gas stream. These cooling processes comprise: Mechanical Refrigeration (MR), Joule Thompson (JT) expansion and Turbo expanders (TE), or a combination (e.g. MR-JT). These NGL recovery processes have been optimised over decades with respect to specific compression duty (i.e. MW/tonne NGL/hr). These optimisations often include: 1) smart exchange of heat between different process streams, 2) different feed trays in the fractionation column and 3) lean oil rectification (i.e. column reflux).
Most sensitive to the specific compression duty is the actual operating pressure of the fractionation column. The higher the operating pressure the lower the specific compression duty, but also the lower the relative volatility between the components of fractionation (e.g. C1-C2+ for a de-methanizer, C2−-C3+ for a de-ethanizer etc.), which results in more trays hence larger column and/or less purity in the overhead stream.
European patent 0182643 and U.S. Pat. Nos. 4,061,481; 4,140,504; 4,157,904; 4,171,964 and 4,278,457 issued to Ortloff Corporation disclose various methods for processing natural gas streams wherein the gas stream is cooled and separated into various fractions, such as methane, ethane, propane, butane and condensates.
A disadvantage of the known cooling and separation methods is that they comprise bulky and expensive cooling and refrigeration devices, which have a high energy consumption. These known methods are either based on isenthalpic cooling methods (i.e. Joule Thompson cooling, mechanical refrigeration) or near isentropic cooling methods (i.e. turbo-expander, cyclonic expansion and separation devices). The near isentropic methods are most energy efficient though normally most expensive when turbo expanders are used. However, cyclonic expansion and separation devices are more cost effective while maintaining a high-energy efficiency, albeit less efficient than a turbo expander device. Using a cost effective cyclonic expansion and separation devices, in combination with an isenthalpic cooling cycle (e.g. external refrigeration cycle) can restore the maximum obtainable energy efficiency.
It is therefore an object of the present invention to provide a method and system for cooling and separating a natural gas stream, which is more energy efficient, less bulky and cheaper than the known methods.